Positive displacement fluid measurement systems can be used to measure a fluid flow rate or volume. For example, dispensing systems can use feedback from a positive displacement fluid meter to control the volume of fluid dispensed. Such control systems can be used in lieu of time-on controls to more accurately dispense precise amounts of fluid. An exemplary positive displacement fluid measurement system is an oval gear meter as described in U.S. Pat. No. 8,166,828; U.S. Pat. No. 8,069,719; and U.S. Pat. No. 7,523,660 each assigned to Ecolab Inc., St. Paul, Minn., the disclosure of each of which is hereby incorporated by reference in its entirety. A typical oval gear meter provides a pair of oval gears positioned within an oval gear chamber such that the gears rotate synchronously. In an oval gear meter, pockets are defined between the rotating oval gears and the inner chamber wall. Typically, fluid does not pass directly between the gears, and the volume of fluid exiting the chamber during each rotation is known. Conversely, the volume of fluid flow through a gear meter can be measured by measuring the number of rotations of the gears. Likewise, flow rate can be determined from the speed with which the gears rotate.
The rotational count and/or speed of rotation of gears can be measured in a number of different ways. For example, a timing gear system can be located external to the oval gear meter to measure the number of rotations of the oval gears and generate an appropriate signal representative of the volume flow rate of the fluid. Oval gear and other positive displacement flow meters utilizing timing gear systems usually have a gear chamber that includes one or more shaft apertures for the shafts coupling the gears to the external timing gears. In other cases, some gear meters instead use a non-contact sensor placed outside a substantially sealed chamber to determine gear rotation within the chamber. For example, non-contact magnetic sensors have been used to measure gear rotation.
Another example of a non-contact sensor is described in U.S. Pat. No. 7,523,660 assigned to Ecolab Inc., St. Paul, the disclosure of which is hereby incorporated by reference in its entirety. As the trigger gear rotates in response to fluid flow, the magnetic field generated by the permanent magnet also rotates. A magnetic sensor such as a GMR sensor (giant magneto resistance effect sensor) senses rotation of the magnetic field and generates an output signal representative of gear rotation. When calibrated against known volume or volumetric flow rate, the rotational count or speed of rotation of the gears respectively, the gears meter can be useful for measuring flow characteristics. One or more GMR sensor elements may be used to monitor rotation of the trigger wheel.